Time-of-Flight Mass Measurements for Nuclear Processes in Neutron Star Crusts

Abstract

We present results from time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory that are relevant for neutron star crust models. The masses of 16 neutron-rich nuclei in the scandium--nickel range were determined simultaneously, with the masses of $^{61}\mathrm{V}$, $^{63}\mathrm{Cr}$, $^{66}\mathrm{Mn}$, and $^{74}\mathrm{Ni}$ measured for the first time with mass excesses of $\ensuremath{-}30.510(890)\text{ }\text{ }\mathrm{MeV}$, $\ensuremath{-}35.280(650)\text{ }\text{ }\mathrm{MeV}$, $\ensuremath{-}36.900(790)\text{ }\text{ }\mathrm{MeV}$, and $\ensuremath{-}49.210(990)\text{ }\text{ }\mathrm{MeV}$, respectively. With these results the locations of the dominant electron capture heat sources in the outer crust of accreting neutron stars that exhibit super bursts are now experimentally constrained. We find the experimental $Q$ value for the $^{66}\mathrm{Fe}\ensuremath{\rightarrow}^{66}\mathrm{Mn}$ electron capture to be 2.1 MeV ($2.6\ensuremath{\sigma}$) smaller than predicted, resulting in the transition occurring significantly closer to the neutron star surface.